Anyone watching a news program can get a basic idea of the difference between an essay and an interview. A television news report usually begins with a reporter presenting a story and providing some background information for viewers, which is the essay portion of the report. Often, an expert or a person who experienced the event being covered is then asked to speak, and the reporter asks that person questions about their experience and asks them to clarify any points that are unclear, which is the interview portion of the report.
The purpose of an essay is to inform. An essay writer gathers as much information as they can on a subject, puts it all together to form an idea about it, and then presents the information to others using their own words and conclusions.
The purpose of an interview is to gather information by getting an expert opinion or capturing a first-person experience.
For an essay, only a writer is necessary, although there are essays on which teams have collaborated to research, analyze, and produce essays together. Most essays, however, have one writer who studies a particular topic, organizes their thoughts, and writes a structured piece that explains the writer's conclusions about that topic.
For an interview, at least two people must be involved. One person asks questions and the other answers them. Interviews can be used as sources for essays. For example, if you were writing an essay on fair banking practices, you may want to interview someone who works in a bank and quote them as backup for your argument or as a source for a factual statement you've written. You can also use interviews just to clarify points on which you are unclear in order to understand your subject better even if you never quote the person you've interviewed directly.
Anyone can write an essay. Quotes from interviews or from other written works can be used in essays as long as the proper citation for that source is included.
For an interview, certain rules around ethics must be applied. No one should conduct an interview without informing the person being interviewed and clearly explaining how the interviewee's words will be used. Publishing someone's words without their permission to use in a written piece, in some cases, can get a writer in a lot of trouble. Make sure to review and fully understand all of the ethical rules around interviewing before conducting one to use in a formal paper.